The invention relates to a procedure for triggering a load element, such as an essentially ohmic load element, using an electronic switching element (S1) in the load circuit, wherein the voltage (Ua) on the load element is controlled with a maximum specified increase.
The triggering, in particular also of many essentially ohmic electric loads in a motor vehicle (e.g. lamps, heating coils, etc.) is achieved using pulse width modulation (PMW). Here, the power delivered to the load can be regulated or controlled, whereby the losses in the control electronics can be kept to a low level using the switching operation.
With the pulse width modulated triggering of loads in motor vehicles, however, electromagnetic fields are emitted via the battery and load wires, which may then interrupt the radio reception in the vehicle.
For this reason, limit values have been specified in different standards (IEC, ISO, CISPR) which reduce the influence on the radio receiver in the corresponding spectra to an acceptable level.
The suppression of the alternating currents on the feed wires is usually achieved using low-pass filters (LC) in the input wires. The size and the cost of the filters depend on the current carrying capacity, the damping required, the quality and the limit frequency. The PWM is normally generated in motor vehicle control units via for a plurality of load elements by timer modules which are integrated into the microcontroller, whereby all PWM outputs switch on at the same point in time, and switch off at different points in time, depending on the PWM pulse-width repetition rate.
This procedure has the disadvantage that a very large alternating current is taken from the network, and that as a result, high demands are required of the filters used, in particular the storage capacitors.
FIG. 1 shows a diagram of a replacement circuit for the switch in PWM mode. Here, any load required (in this example, a filament lamp) is triggered via a switch (such as an N-MOSFET or bipolar transistor) using the signal in a pulse-modulated manner in a switching frequency range of 25 Hz-500 Hz. Depending on the switching condition of the switch, current is either taken from the network or not during the process. The time sequence of the input and output flow is shown subsequently in FIG. 2.
The current gradients which occur generate high-frequency current shares on the feed wires, which themselves cause faults in the LW, MW and SW (150 kHz to 10 mHz) radio frequencies.
Standard procedures dampen the high-frequency alternating currents on the feed wires with filters in the input and output wires, as shown below in FIGS. 3 to 5.
The disadvantage of the filter is its high costs and space requirements, however, which can make the electronics more expensive and fail to miniaturise them (integration in silicon).
A further option for reducing the electromagnetic radiation is to reduce the switching speed in the switch, which enables the high-frequency current shares to be reduced to the required size. Here, however, switching losses with reduced switching speed increase, which are unwanted due to the fact that they heat up the electronic system. FIG. 8 illustrates the switching sequence based on the output voltage with a so-called “constant slew rate”, together with the losses which arise within the “active phase” of the electronic switches. It is already known, for inductive load elements in particular, that the increase of the edge of the switch can be altered in relation to time via the switching sequence, in order to reduce the power loss. Here, however, the edge of the switch is altered according to fixed, specified points in time. In DE 44 13 546 A1, furthermore, with an inductive load with a free wheeling circuit, the edge of the switch is altered in relation to the switching condition of the switch element. This procedure is also known from US-A-2001/0040410.
The object of the invention is to present a procedure of the type initially described, in particular also for ohmic load elements, and to improve the procedure known to date with regard to the switching time, the power loss and the interfering radiation, i.e. to further reduce the interferences, for example, without increasing the power loss, or to reduce the switching time and power loss while maintaining the existing interfering radiation. This object is attained by a method for triggering a load element using an electronic switching element (S1) in a load circuit. The method includes controlling a voltage (Ua) on the load element with a maximum specified increase and recording, during a switching procedure, an effectively occurring power loss or a related value (Ua/Ubat), wherein the effective increase (I1+I2+I3) is controlled dependently on the recorded power loss.
The invention is based on the knowledge that in order to positively influence the high-frequency interference spectrum, it is sufficient to lower the inclination in the area which is non-critical for the power loss at the beginning and the end of the switching procedure, i.e. to work there with a delay which lies below the delay in the active phase, while on the other hand, to utilise a high increase in the active area with high power loss, i.e. a short switching time.
This procedure is oriented in particular on the triggering of an essentially ohmic load element, i.e. with a negligible inductive or capacitive share. Using an electronic switching element in the load circuit, the voltage on the load element is controlled with a maximum specified increase, whereby during the switching procedure, the power loss currently occurring or a value which depends on it is recorded during the switching procedure, and the effective increase is controlled in dependence on it.
This is preferably achieved in such a manner that during the switching procedure, the effective ratio between the voltage on the load element and the supply voltage (Us/Ubat) is recorded, whereby at least for certain value ranges of the ratio (Ua/Ubat), assigned increase values are specified, and the increase is adapted accordingly during the switching procedure.
The increase values can be specified in the form of specific current values, for example, which are fed to a gate in a MOSFET transistor as an electronic switching device.
The present invention influences the edges in the switch element in such a manner, that the build-up speed                is high in the phase in which the power loss is high        is low in the phase in which the power loss is low        
A procedure results for triggering a load element using an electronic switch element in the load circuit, whereby the switching procedure is completed in at least three phases. In the first phase, the output voltage is controlled up to a first specified threshold value (e.g. 15%), with a maximum of one first increase; in the subsequent phase, the output voltage is controlled up to a second specified threshold value (e.g. 85%) with a maximum of one second increase; in the third subsequent phase, it is controlled again with a maximum of the first increase, whereby the second increase is greater than the first increase.
This procedure can preferably be further refined by several intermediate stages in such a manner, for example, that the switching procedure is completed in five phases, whereby                in the first phase, the output voltage is controlled up to a first, specified threshold value (e.g. 10%) with a first increase        in the second phase, the output voltage is controlled up to a second, specified threshold value (e.g. 20%) with a second increase        in the third phase, the output voltage is controlled up to a third, specified threshold value (e.g. 80%) with a third increase        in the fourth phase, the output voltage is controlled up to a second, specified threshold value (e.g. 90%) with the second increase        in the final, fifth phase, the output voltage is controlled up to a supply voltage with the first increase        whereby the third increase is larger than the second increase, which is in turn larger than the first increase        